<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input{
            color:#999;
        }
    </style>
</head>
<body>
    <div>
        <input type="text" value="sh">
    </div>
    <script>
        let ipt = document.querySelector('input')
        ipt.onfocus=function(){
            if(this.value === 'sh'){
                this.value = ''
            }else{
                this.style.color = '#333';
            }
        }
        ipt.onblur = function(){
            if(this.value === ''){
                this.value = 'sh'
            }
            else{
                this.style.color = '#999';
            }
        }   
    </script>
</body>

</html>